The present invention relates to the field of semiconductor and dielectric optical components and devices used for optical transmission or optical digital data processing, in particular to the field of optical mode transformers between two waveguides.
One of the problems of the so called “Silicon Photonics” is the efficient coupling of high-contrast index waveguide, such as Si-waveguides, to the standard single mode optical fiber. Typically, a standard single-mode fiber has a mode field diameter comprised between 8 and 12 μm, more typically of 9-10 μm. The reason of the aforementioned problem lies in the difference in mode size, as well as in the refractive index, between the optical fiber and the Si waveguide.
The difference in mode size prevents to directly connect the single mode fiber to the high-index contrast waveguide, for example through a butt coupling, because the coupling can give rise to a high power loss, due to the difference in the mode size, the latter being typically of at least an order of magnitude.
The difference in refractive index causes a mode velocity difference between the waveguide and the fiber, resulting again in a high coupling loss when a direct coupling is performed.
To enable an efficient coupling between such different optical modes, optical mode converters, known also as optical mode transformers, are made in order to reshape the mode of the optical signal coming from the waveguide/optical fiber and to make its profile compatible with that of the mode guided in the optical fiber/waveguide in which the optical signal should be coupled. This mode matching should be done while preserving the performance characteristics of the optical device, e.g., by keeping the power losses caused by the optical coupling between the waveguide and the fiber below a certain reasonable level.
There have been several approaches to achieve efficient coupling between two different waveguides or waveguide/fiber with different refractive indices.
A known approach is to provide the high-index contrast waveguide with a taper.
U.S. Pat. No. 6,633,705 in the name of Alcatel discloses an optical module which contains at least one optical component whose optical input/output is directed towards an optical fiber to which it is to be coupled. At least one additional optical waveguide is arranged between this optical component and the optical fiber. The optical component is provided with an optical waveguide whose end, facing towards the optical fiber, is tapered or widened for a mode field adaptation. Additionally, the end of each interposed optical waveguide facing towards the optical fiber is also tapered or widened for a modal field adaptation.
US patent application n. 2006/0285797 describes an integrated optical mode transformer which provides a low loss interconnection between an optical fiber and an integrated optic waveguide having a spot size different from that of the fiber. The mode transformer is comprised of two waveguide layers, an upper layer and a lower layer. With the upper layer being contiguous to the lower layer. The lower layer is the integrated optic waveguide layer forming the optical circuit. The input dimension of the composite two-waveguide structure supports a fundamental mode that accepts all of the light present in the optical fiber. The upper waveguide layer is tapered down from an input width to an output width and then terminates in such a way that at the termination substantially all of the input optical power resides in the lower waveguide layer. The two-waveguide layer structure is fabricated by deposition and planarization techniques.
U.S. Pat. No. 6,697,551 in the name of the Massachusetts Institute of Technology is relative to a mode transformer that enables low-loss coupling between optical modes of two waveguides with different index difference. The mode size and effective index are gradually changed between two waveguides to gradually transform the mode shape, size and speed with minimum power loss. The mode transformer is useful for coupling the mode of an optical fiber waveguide with low index difference to the mode of a planar high index difference waveguide, and vice versa.
A different approach is disclosed in U.S. Pat. No. 7,099,540 in the name of NEC Corporation, where an optical coupling device is disclosed, having an optical waveguide varied in difference in specific refractive index between a clad layer and a core layer. The optical waveguide serves as a convergent/divergent spot size converter, since reduction in cross section of the core layer is not required for the convergent/divergent spot size converter so that the optical coupling device is produced at high production yield.
US patent application No. 2005/0185893 describes an apparatus and a method for reducing a mode size of an optical beam. In one embodiment of the described solution, the apparatus includes a first optical waveguide disposed in a first semiconductor material of a semiconductor layer. The first optical waveguide includes an inverted tapered inner core disposed in an untapered outer core of the first optical waveguide. The inverted tapered inner core includes a smaller end and a larger end. The apparatus further includes a second optical waveguide disposed in a second semiconductor material of the semiconductor layer. The second optical waveguide is a tapered optical waveguide having a larger end and a smaller end. The larger end of the second optical waveguide is disposed proximate to the larger end of the inverted tapered inner core of the first optical waveguide such that an optical beam is to be directed from the smaller end to the larger end of the first optical waveguide from the larger end to the smaller end of the second optical waveguide.
In “TE-TM coupling of a standard fiber to a Si-wire waveguide”, written by Raffaella Costa et al., and published in the proceedings of the European Conference on Integrated Optics (ECIO), Copenhagen, Denmark, 25-27-Apr. 2007, light coupling from a small core fiber to a Si-wire waveguide is demonstrated for both TE and TM through a vertical coupler. A polarization independent coupling is said to be realized as a first trial with an efficiency of 72%. The small core fiber to Si-wire coupling is performed through an intermediate waveguide having a squared-shaped core with Δn=4.5% vertically coupled by means of a silicon taper. The Si-wire core is 220 nm thick.